Mutations induced by chemical agents and radiation can produce harmful effects in humans and play a role in the multistepped process of carcinogenesis. For example, it is known that base substitutions can activate human cellular ras genes into oncogenes and that at least some carcinogen-induced anchorage independent human fibroblasts express activated ras genes. However, oncogene activation, as well as other changes which are involved in carcinogenesis, can occur by multiple mechanisms and much less is known about the role of specific DNA changes in those processes. For the past decade, we have been studying the mutagenic effect of carcinogens in diploid human fibroblasts and the role of DNA repair and replication in that process. During the past 2.5 yrs, we have been determining at the sequence level the specific kinds of mutations induced when a shuttle vector containing carcinogen adducts replicates in human cells. We now propose to make use of molecular biology techniques to examine the specific kinds of mutations induced by carcinogen-treatment (and other mutagenic processes) in the DNA sequence of the resident HPRT gene in human cells and compare the results with what we and others have found using the shuttle vector. We will determine the kinds of mutations induced by selected carcinogens in the HPRT gene of normal, repair-proficient, human cells, and compare the results with those obtained with these same agents when a carcinogen-treated shuttle vector replicates in such cells. We will also determine the spectrum of mutations induced by these same agents in DNA repair-deficient XP cells. We will determine if the spectrum of the mutations induced in the HPRT gene of normal cells that are exposed to carcinogens just as the gene is to be replicated differs from that found in cells that are exposed approximately 15 hr prior to S-phase and allowed time to repair specific types of DNA lesions. Since XP variant cells are significantly more sensitive to UV mutagenesis than normal cells, but have a normal rate of excision repair, we will compare the spectrum of UV mutations formed in the HPRT gene of XP variant cells to that of normal cells and determine the kinds of mutations induced in a UV-irradiated shuttle vector which replicates in XP variant cells compared to that already determined when the vector replicates in normal and repair- deficient XP cells. The polymerase chain reaction method will be used to isolate the HPRT gene. These studies should provide basis information on human cell mutagenesis,including whether the mutagenic mechanisms operating during extrachromosomal replication differ from those operating on resident cellular genes.